The present invention relates to a new and improved method of, and a drafting, arrangement for, spinning machines, particularly for draw frames, for processing a fiber sliver with a staple range encompassing short to long staple fibers.
The drafting arrangement comprises a pre-draft zone and a main draft zone, as well as bottom rolls arranged on a machine frame and pressure rolls arranged thereabove and forming drafting zones.
In German Pat. No. 1,250,315 a drafting arrangement is shown and described, which, as seen in the direction of movement of the fiber sliver, contains an input pair of rolls, an intermediate pair of rolls, and a pair of delivery rolls. Each roll pair consists of a bottom roll and a corresponding or related pressure cell.
The pair of input rolls and the pair of intermediate rolls collectively form a pre-draft zone, whereas the pair of intermediate rolls and the pair of delivery rolls form a main draft zone.
The pair of input rolls is linearly shiftable forwards and backwards, as seen in the direction of movement of the fiber sliver, for the purpose of adapting the length of the pre-draft zone. Both rolls are independently shiftable.
The intermediate and the delivery bottom rolls are fixedly arranged, whereas the intermediate and the delivery pressure rolls are linearly shiftable in the same manner as the input pressure roll.
All three pressure rolls are vertically movable with respect to a base plate taking-up the bearing blocks of the bottom rolls, in such a manner that during the aforementioned linear shifting of the pressure rolls, the latter can effect, in combination with the vertical movability, a movement about the fixed or fixedly arranged bottom rolls. Thus, the possibility is given to adapt the length of the main drafting zone and to adapt it to the staple length to a certain extent.
The diameter of the delivery bottom roll is larger than the diameter of all of the other rolls. The bearing blocks of the pressure rolls are arranged to be linearly shiftable on the bearing blocks of the bottom rolls. The pressure applied to the pressure rolls is exerted by using spring-loaded pressure pistons which are mounted upon a pivotable and arrestable support member.
Furthermore, in Swiss Pat. No. 426,570 there is disclosed a drafting arrangement containing a pre-draft zone and a main draft zone, in which the pressure rolls are arranged to be pivotable about the axis of the bottom rolls for enlarging or shortening the wrapping arc of the fiber sliver upon the bottom rolls.
For adapting the nip line distances, limiting the pre-draft zone and the main draft zone, to the staple length of the fiber material to be processed, the mutual distances of the groups of rolls are changeable.
Production increases in a drafting arrangement necessarily imply an increase in sliver speed. High sliver speeds, for instance, of 800 m/min. or more, of the drafted sliver, i.e. at the delivery side of the drafting arrangement, require high rotational speeds of the drafting arrangement rolls which, in turn, imposes more stringent requirements upon the bearings of the rolls.
The useful service life of a bearing is determined, apart from the factors of rotational speed and bearing load, by the accuracy of the settings or mounting, for instance with respect to the parallelity of the pressure rolls and the related bottom rolls, and with respect to the accurate alignment of the roll axes with respect to the elements driving the shafts.
If the drafting arrangement disclosed in German Pat. No. 1,250,315 is considered under the abovementioned aspects, it will be recognised that for the shiftability and the arrestability, respectively, of the bearing blocks for the pair of input rolls, and the bearing blocks for the intermediate and the delivery pressure rolls, there are not provided any special devices or facilities for accurately arresting or fixation thereof. Hence, these bearing positions are only adjustable or settable in a relatively inaccurate manner, or only by using special setting or adjustment devices, which have been neither shown nor described. The use of such auxiliary devices, however, is time-consuming, cumbersome, and thus, unsatisfactory.
Furthermore, the mutual linear shiftability of the pressure rolls with respect to the bottom rolls, for the purpose of adapting the nip line distances of the drafting zones, exhibits the disadvantage that, due to the linear shifting of the pressure rolls the spring or resilient forces of the pressure pistons act with varying force upon the fiber sliver, depending upon the position of the pressure rolls at the bottom rolls, which influences the force components in angular direction.
A further disadvantage resides in the large diameter of the delivery bottom roll, which causes an increased nip line distance in the main drafting zone.
Furthermore, the drafting system exhibits the disadvantage that, for instance, for guiding the fiber sliver into a sliver can, there is required an additional deflection of the sliver after the drafting arrangement. At sliver speeds of 13.3 m/sec. and more such imposes an additional, undesirable stress upon the sliver, caused by centrifugal forces, and thus, constitutes a disadvantage of the method.